Crosslinked polyethylene oil filled high voltage powered cable



United. States Patent 3,527,874 CROSSLINKED POLYETHYLENE OIL FILLED HIGHVOLTAGE POWERED CABLE Toshiyuki Hayami, Osaka, Japan, assignor toSumltomo Electric Industries, Ltd., Osaka, Japan, a company of Ja an FFiled Sept. 14, 1967, Ser. No. 667,778 Claims priority, applicationJapan, Sept. 27, 1966, 41/90,009, 41/90,010 Int. Cl. H0111 7/34, 9/36U.S. Cl. 174-24 3 Claims ABSTRACT OF THE DISCLOSURE The presentinvention relates to an improvement in crosslinked polyethylene highvoltage cables.

With crosslinked polyethylene high voltage power cable coresmanufactured by the extrusion process, it is not possible to completelyeliminate gaps and injuries on the interface between the conductor andthe insulation, or small voids formed in the insulation, or foreignbodies which find their way into the cable, etc., so that there maydevelop electrically weak points where the corona effect takes placeupon application of a high voltage to the cable and, as a result, causesinsulation breakdown.

The present invention is directed to the provision of high voltage powercables of excellent electrical properties, preventing the generation ofcorona at such electrically weak points.

This invention is characterized in that the core is provided with acrosslinked polyethylene insulating layer, and the interfaces of theconductor and this insulating layer are maintained filled with siliconoil. A semi-electroconductive layer is provided on the outside of theinsulating layer.

A second characteristic of the present invention is as follows: Sincesilicon oil can hardly be absorbed in crosslinked polyethyleneinsulation, it may fail to give sufficient reinforcement to the weakpoints caused by voids and foreign bodies in the insulation, even thoughit can reinforce the weak points on the interfaces of the insulation andthe conductor. n the other hand, hydrocarbon insulating oils such aspolybutane, etc. are not good for practical use in cables because theycause crosslinked polyethylene to remarkably swell. In view of theabove, it is the object of the present invention to obtain cables whichare excellent both electrically and mechanically by giving full displayto the merits of the two kinds of oil through mixing of a hydrocarbonoil and silicon oil at a certain ratio.

That is to say, the present invention is characterized in that whenprovision is made for a cable core having a central conductor and aninsulating layer, the interfaces of the conductor and the insulatinglayer are filled with a mixed oil consisting of silicon oil andhydrocarbon oil and a semi-electroconductive layer is provided on theoutside of the insulating layer.

Other objects and advantages appear hereinafter in the followingdescription and claims.

The accompanying drawings show, for the purpose of exemplificationwithout limiting the invention or the Patented Sept. 8, 1970 "ice claimsthereto, certain practical embodiments illustrating the principles ofthis invention wherein:

FIG. 1 is a sectional View of a power cable embodying the embodiments ofthe present invention.

FIG. 2 shows the V-t characteristic curves of the cable of thisinvention and that of a conventional cable without oil.

FIG. 3 shows the V-t characteristic curve of another embodiment of thepresent invention.

In FIG. 1, there is shown a hollow conductor 1 having an oil passage inits interior, and a cable insulating layer 2 formed integrally on theconductor by extruding crosslinked polyethylene insulation over theconductor. A semielectroconductive layer 3 is placed on the outside ofthe cable and is formed by extruding semi-electroconductive polyolefinerendered electroconductive, for example by addition of carbon black. Ametallic shielding layer 4 is made by winding copper tape or extrudinglead, aluminum or the like over the semi-electroconductive layer 3. Anouter anti-corrosion layer of polyvinyl chloride, chloroprene or thelike is provided over the shielding layer 4. Silicon oil 6 is providedin the interior of the conductor 1.

The method of manufacturing the cable of the present invention may be asfollows: An uncrosslinked polyethylene insulating layer 2 containing acrosslinking agent, such as di-a-cumyl peroxide (-D.C.P.), is extrudedonto a conductor to cover the same, and crosslinking is caused to takeplace at a high temperature under high pressure, such as, respectively,150 C. or higher and 10 atmospheric pressures or more. An outer sheathis then applied over cable cores made in the above-mentioned way, andthen silicon oil is poured into one end of the conductor. For pouringthe oil into the conductor, it is preferable to first previouslyevacuate the conductor interior with the object of filling the whole ofthe cable interior with the silicon oil. The semi-electroconductivelayer 3 on the outside may be extruded together with the insulatinglayer 2 or may be applied after the insulating layer 2 has beencrosslinked.

It is not always necessary that the conductor be a hollow conductor. Adecision as to whether a hollow conductor or an ordinary conductorshould be used should be made after considering the cable size, length,etc. The essential requirement is that silicon oil be present at alltimes between the conductor and the insulation.

As to the oil pressure regulating device, a suitable one may be selectedafter considering the installation condition of the cable, the quantityof the oil, etc. In some cases no such device is necessary.

FIG. 2 shows a comparison between the V-t characteristic curve of thecable of the present invention and that of a crosslinked polyethylenecable without oil filling. The specimens are crosslinked polyethylenecable, 100 mm. with an insulation thickness of a 3 mm., Curve A is forthe cable of the present invention and the curve B for the cable withoutoil filling.

The degree of swelling of the crosslinked polyethylene, the insulationlayer 2 for the cables of the present invention, was measured byimmersing it in silicon oil having a viscosity of 10 centistokes heatedto weather temperature, C., C., C. and C. According to the results, thedegree of swelling was 0.2% or less even in the case where the oil washeated to 150 C. (The limit for weight ratio measurement is 0.2% ormore.)

It was also found that the electrical properties (e tan 6 p) ofcrosslinked polyethylene immersed in silicon oil for one month remainedentirely unchanged as compared with the same properties beforeimmersion. In the above formula e stands for the dielectric constant,

tan 8 stands for the dielectric tangent and p the specific volumeresistivity.

crosslinked polyethylene immersed in various oils is shown in Table IIbelow.

TAB LE II Hydrocarbon oil (polybutane. Silicon oil Silicon oil insulatmg 011 for I containing 3% containing oil-filled cables) Sihcon oilhydrocarbon oil hydrocarbon oil Degree of swelling at 80 0.... 2-3 timesAlmost no swell- 3 10 }ng)(0.2% or ess The construction according to thepresent invention being as described above, silicon oil fills up thinlayer gaps which may be present due to poor adhesion between conductorand insulation, scars which may exist on the conductor, etc., so thatcorona effect does not take place in interface between the conductor andthe insulating layer. Cables having good properties with respect to highvoltage applications are thus obtained, and it is possible to make thedesign stress higher than in the case of plastic cables without oil. Thecables will have a longer life for high voltage use. Furthermore, assilicon oil is used, the insulation is not swollen due to the oil evenat a high temperature, so that thermally stable cables may be obtainedin the case of the present invention. Also, as silicon oil has almostthe same dielectric characteristics as crosslinked polyethylene, thedielectric properties of the insulation are not impaired by theintroduction of the oil. Thus it is possible to obtain cables havinggood dielectric properties of dielectric constant (e), dielectrictangent (tan 6), etc. The cables have also a great advantage of a verylarge current carrying capacity, the product of dielectric constant anddielectric tangent being 1/ 10-1/ 30 times that of oil-filled papercables. It

is well known that crosslinked polyethylene having netted molecularstructure as a result of molecular cross-linking has a greaterresistance to heat than polyethylene. In the case of oil-filled cableslike that of the present invention, the property to resist environmentalstress cracking (E.S.C.) due to ambient medium is a problem. However,there is no need for concern in this connection because crosslinkedpolyethylene is far superior to polyethylene in this respect. As aresult it enhances the dependability of the cables.

The Table I below shows the result of tests to compare the E.S.C. ofpolyethylene and crosslinked polyethylene.

mens cracked. Crosslinked polyethylene (above 300 hours: 10 of 10specimens free 1polyethylene subjected to crossfrom cracking.

lllfilng.

The tests were made by immersing in silicon oil of 50 C. the specimensof polyethylene and crosslinked polyethylene which had been bent to acertain shape and measuring the time that passed before the specimenscracked. From the test results shown in the above Table I, it can beseen that crosslinked polyethylene has excellent resistance to E.S.C.

As described above, the cable made according to the present inventionhas excellent dielectric characteristics and has a good stability, itbeing possible to take a higher design stress than in the case of theplastic cables heretofore in use. It is therefore possible to apply theinvention to high voltage power cables of 275 kv. or 500 kv. class.

Another embodimentof the present invention involves the filling of thecable conductor with a mixed oil of silicon oil and a hydrocarbon oilinstead of silicon oil used in the first embodiment. The degree ofswelling of FIG. 3 shows a comparison of the V-t characteristic ofcrosslinked polyethylene, 3 mm. thickness and mm. Curve A is for thecable according to the present invention filled with silicon oil with 5%hydrocarbon oil, curve B for the cable filled with silicon oil only, andcurve C is for the cable without any oil filling. From the results shownin FIG. 3, it can be seen that the cable according to the presentinvention having curve A is superior toB and C with respect to the Vtcharacteristic.

As previously explained, the cables of the present invention have betterelectrical properties because reinforcement can be provided not only inconnection with electrically weak points on the interface between theconductor and the insulation but also in connection with electricallyweak points caused by bubbles and foreign bodies which are liable totake place in the insulation by providing a mixture of silicon oil andhydrocarbon oil to continually maintain on the conductor side of acrosslinked polyethylene cable formed in one body by an extruder.

. The quantity of a hydrocarbon oil added to silicon oil is preferablybetween 1% and 30%, inclusive.

I claim:

1. In a high voltage cable having improved dielectric properties, aconductor, a crosslinked polyethylene insulating layer extruded on saidconductor, said cable characterized by an insulation oil mixture ofsilicon oil with a hydrocarbon oil content in the range of 1 to 30percent supplied interior of said insulating layer to maintain said oilmixture in continuous attendance at the interface between said conductorand the inner surface of said insulating layer to prevent the generationof corona.

2. The high voltage cable of claim 1 characterized by asemi-electroconductive layer extruded over said insu lating layer.

3. The high voltage cable of claim 1 characterized in that saidconductor is stranded and has a hollow interior to receive saidinsulation oil mixture.

References Cited UNITED STATES PATENTS 3,077,510 2/1963 Olds 174-253,244,800 4/1966 McLaughlin et al. 174-110 3,329,764 7/1967 Tanges 174743,297,819 1/1967 Wetmore 174l27 3,078,333 2/1963 Kang 174-26 X 2,377,6896/1945 Hyde l7425 X FOREIGN PATENTS 697,963 11/1964 Canada.

940,269" 10/ 1963 Great Britain.

958,418 5/ 1964 Great Britain.

OTHER REFERENCES Insulating Materials For Design and EngineeringPractice, John Wiley & Sons, Inc., 1962, pp. 241-243.

LARAMIE E. ASKIN, Primary Examiner A. T. GRIMLEY, Assistant Examiner USCl. X.R. 174-15, 102,

